In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called ‘wet’ lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called ‘driographic’ printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a is photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Typical photopolymer plates are sensitized for visible light, mainly for exposure by an Ar laser (488 nm) or a FD-YAG laser (532 nm). The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material. An advantage of violet plate-setters, compared to visible light plate-setters, are the improved safe-light conditions. Laser diodes, emitting violet light with a wavelength around 405 nm (±15 nm) are at present the most important, commercially available, violet laser diodes. Photopolymer plates generally contain a polymerizable monomer, a binder, a photo-initiator and a sensitizing dye. EP-A 985 683, EP-A 1 048 982 and EP-A 1 070 990 disclose a composition comprising a titanocene compound as photo-initiator and specific dyes as sensitizers for the wavelength range from 350 to 450 nm. EP-A 1 035 435 discloses a 1,3-dihydro-1-oxo-2H-indene derivative as sensitizing dye. A wide range of dyes for the wavelength range from 300 to 1200 nm is disclosed in EP-A 1 091 247.
To enable short exposure times with the commercially available blue or violet laser diodes, resulting in a higher throughput (i.e. higher numbers of printing plate precursors that can be exposed in a given time interval) there is a need to increase the sensitivity of the violet sensitive photopolymerizable compositions. EP-A 1 349 006 and WO2005/029187 disclose a photopolymerizable composition using optical brightheners, e.g. distyrylbenzenes, as sensitizers, suitable for exposure with violet laser diodes. In EP-A 1 621 928, a composition is disclosed which is photopolymerizable upon absorption of light in the wavelength range from 300 to 450 nm, the composition comprising a binder, a polymerizable compound, a sensitizer and a photo-initiator, wherein the sensitizer is a fluorene compound that is conjugated via a double or triple bond with an aromatic or heteroaromatic group. Also in EP-A 1 591 242 and 1 688 792, distyryl benzenes are disclosed as efficient sensitizers for the wavelength range of from 300 to 450 nm.
The photopolymerizable compositions described in EP-A 1 349 006, WO2005/029187 and EP-A 1 621 928 are high enough in sensitivity to enable exposure with violet light having an energy density, measured on the surface of the plate, of 100 μJ/cm2 or less.
A problem associated with the above cited sensitizers may be critical crystallization behaviour in the photopolymerizable layer. The formation of crystals may locally decrease the polymerization efficiency, resulting in areas in the imaged parts of the precursor partly or completely removed during development. Since these areas may not sufficiently take up ink during printing, a poor, sometimes unacceptable, printing quality may be obtained. The occurrence of these crystallization defects may become more pronounced when the printing plate precursor is stored before exposure and development, i.e. the storage stability of the precursor is poor.
To avoid the occurrence of crystallization of the sensitizer WO2005/029187 proposes the use of a sensitizer having a solubility in methylethylketone (MEK) of at least 15 g/kg measured at 20° C. Highly preferred sensitizers according to WO2005/029187 are distyrylbenzenes having branched substituents on the benzene rings. However, printing plate precursors comprising these sensitizers may still have a poor storage stability, which may result in printing plates having a poor printing quality due to crystallization defects.